Electrical systems for a number of known reasons may experience unwanted electrical arc faults. One type of arc fault is a parallel arc fault, which can occur as a short circuit or a ground fault. A short circuit arc decreases the dielectric strength of insulation separating the conductors, allowing a high-impedance, low-current arc fault to develop that may carbonize the conductor's insulation, thereby further decreasing the dielectric of the insulation separating the conductors. The result may be increased current and arcing in the circuit. The current flow in a short circuit, parallel arc fault may be limited by the system impedance and the impedance of the arc fault itself.
Current methods of detecting such faults examine the peak current magnitude of a half cycle and the current slope at the zero crossing of the half cycle of the waveform. If the slope and amplitude are both above a certain fixed threshold magnitudes, then a parallel arc fault is indicated and the breaker (e.g., an AFCI) may be tripped. However, such threshold levels need to be set relatively high so that certain circuit conditions, which mimic parallel arcs, such as an in rush condition at startup of a motor connected as a load on the circuit protected by the AFCI, do not trip the circuit breaker.
For example, when the current is non-sinusoidal, such as exhibited by nonlinear loads having a high crest factor, such as exhibited by vacuum cleaners, drills, and other household items, the current magnitude at startup might be high enough in an in rush condition to exceed the amplitude threshold. Similarly, such high crest factor loads have a generally higher slope than sinusoids at the same magnitude. Accordingly, such in rush conditions may also exceed the fixed slope thresholds.
Therefore, a need exists for a circuit breaker and detection method that may discriminate between actual arc faults and conditions mimicking an arc fault, such as in rush conditions.